The effect of intensive physical training on cardiac autonomic variability - factors that may influence the results

Abstract:

The study dealt with the influence of exercise on the autonomic nervous system (ANS) and with factors that may influence the results. ANS function was measured in 183 young, healthy participants, before and after a twelve week standardised, medium-to-high volume physical training programme, in a controlled environment. The effects of the training programme were assessed on resting ANS functioning, during standing and on the response to an orthostatic challenge. ANS function was assessed by means of heart rate variability (HRV) determination. HRV was quantified by three different analytical techniques, i.e., time domain analysis (RR, STDRR, RMSSD and pNN50), frequency domain analysis (LF,LFnu, HF, HFnu and LF/HF) and Poincaré plot analysis (SD1 and SD2). The influence of technical variations, such as variations in tachogram length and period of recording, as well as the influence of pre-intervention values of physiological variables, such as blood pressure, BMI, VO2 max and ANS functioning, on the response to the exercise intervention, were assessed. Results on the exercise intervention showed: <ul><li> Increased supine, as well as standing, parasympathetic cardiac control as indicated by time domain, frequency domain and Poincaré analyses. </li><li> Decreased sympathetic control in the supine position and increased sympathetic control during rising and standing. </li><li> Increased vagal withdrawal, as well as increased sympathetic control during the first phase of the orthostatic response to rising from the supine position. </li><li> Only an exercise-induced increase in sympathetic control when the orthostatic response was measured as the difference between standing and supine. </li></ul> Results on exercise-induced changes in sympathetic and parasympathetic ANS control differ, depending on posture. It is suggested that the effects of an exercise intervention on sympathetic and parasympathetic ANS control of the heart should be assessed from measurements in the supine, in the standing, and in response to an orthostatic stressor. It is further suggested that information obtained during rising will give additional information on the response of the ANS. This study showed that technical as well as physiological variations may lead to differences in the outcome of HRV studies. Results from the technique evaluation showed that the length and period of tachogram recordings should be standardised, especially during an orthostatic challenge. Starting the recording too late will miss out on the initial response to a change in body position. Longer recording times will represent the mean of HRV values obtained during the orthostatic response and that obtained after stabilisation in the standing position. Investigations into the influence of pre-intervention physiological status on exercise-induced changes showed: <ul><li> Baseline ANS functioning is a significant contributor to variations in the ANS response to an exercise intervention. </li><li> Pre-intervention values for physiological variables, such as blood pressure, BMI and VO2 max do not have a significant influence on the HRV response to exercise in young, healthy individuals of average fitness</li><li> Regression analyses confirm the correlation results, i.e. that baseline ANS function is a significant predictor of the ANS response to exercise. </li><li> However, regression results indicated that the combination of pre-intervention values for LFms2, HFms2, BMI, VO2 max, gender and blood pressure, contributes only between 12.83% and 29.82%, depending on the HRV variable, to the exercise induced changes in the autonomic nervous system. </li></ul>